Download Jovian system Massive, gas giant planet, ~0.001 Solar masses

Jovian system
Massive, gas giant planet,
~0.001 Solar masses
Europa: radius 1560km
Orbital radius: 5.2 AU - distance beyond snowline where
rock and water ice dominates
4 large Galilean moons:
• Io (period 3.55 days)
• Europa
• Ganymede
• Callisto
1:2:4 resonance between
the orbital periods of the
inner 3 satellites
Gradient of mean density:
• Io 3530 kg/m3
• Europa 3018 kg/m3
• Ganymede 1936 kg/m3
• Callisto 1851 kg/m3
Extraterrestrial Life: Spring 2008
Surface of Europa is made of
water ice
density of rock
(silicates)
mixture of rock plus
lower density ice
(~920 kg/m3)
Extraterrestrial Life: Spring 2008
Evidence for an ocean on Europa
(1) Global surface appearance
Mean density would be consistent
with a 6% fraction of water by
mass:
e.g. 1450km rocky interior
with a 110km thick layer of
water (liquid or ice) at the
surface
Europa surface has very few impact
craters - only 3 with diameter > 3km
Young surface
Cratering rate in the outer Solar
System is not well determined:
models suggest an age between
10 million and 1 billion years
Resurfacing must involve melting
at least some of the ice
Extraterrestrial Life: Spring 2008
Extraterrestrial Life: Spring 2008
(3) Magnetic field measurements
(2) Local surface features
Jupiter has a strong magnetic field, which Europa is
orbiting through. Magnetic fields are generated by
currents - flows of charged particles.
Galileo images of the
surface show blocks
of ice, km-10s km in
size, separated by
ridges
If Europa’s interior does not conduct electricity,
the moon cannot generate its own magnetic
field and the field measured near the surface
should be that of Jupiter alone
Crust has been
fractured and then
refrozen in place
Appearance resembles pack ice floating on the ocean
at the North Pole and off the coast of Antarctica
Extraterrestrial Life: Spring 2008
If Europa’s interior conducts electricity, then
the motion of the moon through Jupiter’s field
induces a current, which distorts the Jovian
field near the moon.
Extraterrestrial Life: Spring 2008
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What could the conductor be?
Rock and ice are both poor conductors
Galileo measurements
during a close flyby
Prediction based on Jupiter’s
magnetic field if Europa has
no field of its own
Conclude that Europa does possess its own magnetic
field, and hence that some part of the interior of the
moon must be an electrical conductor…
A salty ocean beneath the ice would be a good conductor:
explains the Galileo magnetic field measurements quite
well
An iron core would also be a good conductor: can’t exclude
this possibility and if one exists, analysis is much more
complicated…
Extraterrestrial Life: Spring 2008
Extraterrestrial Life: Spring 2008
Heat source for Europa
Internal heat of small bodies (Moon, Mars, Mercury) is not
enough to sustain geological activity for the life of the Solar
System
tidal
lag
Tidal heating
Tides are raised on Europa by the different gravitational
force from Jupiter on the near / far side of the moon
Moon is distorted (very slightly) by tidal gravitational force
Stress and try to crack the surface
Dissipation of the tidal energy results in heat in the
moon’s interior
As it rotates, tidal bulges don’t quite align with the axis
between the Moon and Jupiter
Dissipation of the tides leads to heating
Extraterrestrial Life: Spring 2008
Io: moon closest
to Jupiter
Very strong tides
lead to volcanic
activity on the
surface
Extraterrestrial Life: Spring 2008
Possible oceans
Current data provides little constraint on the thickness
of the ice layer overlying the ocean. Two models:
(1) Thin ice layer (~1km), fractured by tidal forces
as Europa orbits Jupiter, or local heating from
volcanic events at the silicate / water boundary
(2) Thick ice layer (at least 20km) fractured by
convection in the ice heated by tides
Extraterrestrial Life: Spring 2008
Extraterrestrial Life: Spring 2008
2
If Europa indeed has an ocean, are the basic conditions
for life met?
• Water: yes
• Carbon: yes - surface of the other Galilean moons
show abundant impact craters… meteorites
on Earth deliver organic material and the same
must be true for Europa
• Energy: possibly… unknown whether hydrothermal
vents or volcanic activity occurs at the bottom
of the ocean
If energy is available, conditions in Europa’s ocean
(T ~ 0C, probably quite salty water) fall within the range
for which extremophiles survive on Earth…
Extraterrestrial Life: Spring 2008
Europa exploration
No current spacecraft in the Jovian system, and no funded
mission to return by NASA or ESA
High priority for “outer planets” science, but expensive and
difficult to fly to Europa (distance, low Solar flux, radiation
environment near Jupiter)
Europa orbiter could definitively establish the presence
or absence of an ocean, and measure the depth of the
ice
Lander could sample the ice near the surface, and
conceivably drill through to the ocean
Extraterrestrial Life: Spring 2008
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